Device and method for determining the state of ageing of an exhaust-gas catalytic converter

ABSTRACT

Device and method for determining the state of ageing of an exhaust-gas catalytic converter wherein an oxygen sensor having an oxygen-sensitive region and a temperature-sensitive region can be actuated in such a manner that a temperature measurement and, as an alternative, an oxygen partial pressure measurement can be carried out. There is provision for an electrical conductivity of a conductor structure of the oxygen sensor to be determined and for an exhaust-gas temperature to be determined therefrom. From the sensor measurements, the extent of exhaust gas catalytic converter ageing may be assessed.

[0001] This application claims the priority of German patent application102 48 842.8, filed Oct. 19, 2002, the disclosure of which is expresslyincorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] The invention relates to a device for determining the state ofageing of an exhaust-gas catalytic converter, and to a method used forthis purpose.

[0003] German Patent DE 41 12 479 C2 (corresponding to U.S. Pat. No.5,303,580) describes a method in which the state of ageing of anexhaust-gas catalytic converter is determined with the aid of an oxygensensor connected to an electronic control unit. The sensor has anoxygen-sensitive region for measuring the oxygen partial pressure in theexhaust gas. The state of ageing of the catalytic converter isdetermined by the control unit by means of this measured variable.

[0004] By contrast, it is an object of the invention to provide a deviceand a method which allow reliable determination of the state of ageingof an exhaust-gas catalytic converter using simple apparatus.

[0005] According to the invention, this object is achieved by a devicehaving an oxygen sensor which is arranged in the exhaust pipe and isassigned to the exhaust-gas catalytic converter, the oxygen sensorhaving an oxygen-sensitive region for measuring an oxygen partialpressure in the exhaust gas and being connected to an electronic controlunit, characterized in that the oxygen sensor has atemperature-sensitive region and can be actuated by the control unit insuch a manner that a temperature measurement and/or an oxygen partialpressure measurement can be carried out.

[0006] The device according to the invention is distinguished by thefact that the oxygen sensor has a temperature-sensitive region and canbe actuated by the control unit in such a manner that a temperaturemeasurement and, as an alternative, the oxygen partial pressuremeasurement can be carried out. The temperature-sensitive region is inthis case at least partially in contact with the exhaust gas. The extentof release of heat of reaction brought about by the catalytic convertercan be determined by means of the temperature measurement. Furthermore,the sensor, via the oxygen-sensitive region, can determine the change inthe oxygen content in the exhaust gas which is brought about by thecatalytic converter. The determination of the two measured variables,temperature and oxygen content, can be carried out as alternatives. Itis preferable for the temperature measurement to be carried out when theinternal combustion engine is warming up and for the oxygen partialpressure measurement to be carried out when the internal combustionengine has warmed up. The two measured variables are characteristic ofvarious performance features of the catalytic converter, so that itsperformance and state of ageing can be conclusively ascertained. Withregard to the determination of the state of ageing of the catalyticconverter, the temperature measurement is preferably used to determinethe light-off temperature of the catalytic converter. In the presentcontext, the term light-off temperature is understood, as is customary,to mean the temperature at which the catalytic converter reaches asignificant catalytic activity, for example 50% conversion. A lowlight-off temperature is generally desirable, but this may rise over thecourse of time during which the catalytic converter is used as a resultof ageing. The measurement of the oxygen partial pressure is preferablyused to determine the oxygen storage capacity of the catalyticconverter. The oxygen storage capacity is likewise subject to ageing.Since the invention allows both the light-off temperature and the oxygenstorage capacity of the catalytic converter to be determined, the stateof ageing of the catalytic converter can be determined comprehensivelyand reliably. Furthermore, the measurement of the oxygen partialpressure is used to control the air/fuel ratio (λ) of the air/fuelmixture supplied to the internal combustion engine. Therefore, theoxygen sensor performs a dual function, so that the device can be ofsimple design.

[0007] In one embodiment of the invention, the temperature-sensitiveregion of the oxygen sensor is formed by its oxygen-sensitive region andis designed in particular as a solid electrolyte. With thisconfiguration, the invention can be realized by means of anelectrochemical sensor. The solid electrolyte is used on the one hand tomeasure the temperature and on the other hand, as an alternative, tomeasure the oxygen partial pressure in the exhaust gas, with the samesensor part being used for both jobs. It is preferable for theelectrical conductivity of the solid electrolyte to be evaluated inorder to measure the temperature and for the electromotive force of theNernst voltage of the solid electrolyte to be evaluated for the purposeof measuring the oxygen partial pressure. This dual function of thesolid electrolyte or the oxygen-sensitive region makes it possible todispense with additional sensor components, resulting in a simple designof sensor.

[0008] In a further embodiment of the invention, thetemperature-sensitive region of the oxygen sensor is designed as aheating conductor structure. In this way, the heating conductorstructure which is generally already present in an oxygen sensor isadvantageously used to measure the temperature. It is preferable for theelectrical conductivity of this heating conductor structure to be usedfor the temperature measurement. For this purpose, the material used forthe heating conductor structure can be a material which has a relativelyhigh temperature coefficient of its electrical conductivity, so that asubstantial measuring effect is achieved. With this configuration of theinvention, it is likewise possible to dispense with additional sensorcomponents and a simple design of sensor likewise results.

[0009] In a further embodiment of the invention, a temperature probe isprovided in the exhaust pipe, and the temperature probe and the oxygensensor are arranged in such a manner in the exhaust pipe that at least apartial region of the exhaust-gas catalytic converter is located betweenthe oxygen sensor and the temperature probe. This makes it possible tomeasure a local temperature difference, so that the amount of heatrelease caused by exothermic reactions in the catalytic converter regioncan be determined particularly reliably. Consequently, it is alsopossible for the light-off temperature of the catalytic converter and/orits ageing-induced deterioration to be determined with particularreliability.

[0010] In a further embodiment of the invention, the oxygen sensor isarranged in the exhaust-gas catalytic converter or in the exhaust pipedownstream of the exhaust-gas catalytic converter, and a second oxygensensor is arranged in the exhaust pipe upstream of the exhaust-gascatalytic converter. This makes it possible to measure a localdifference in the oxygen partial pressure in the exhaust gas, so thatthe oxygen storage capacity of the catalytic converter and/or itsageing-induced deterioration can be determined reliably.

[0011] The method according to the invention is distinguished by thefact that as the internal combustion engine is warming up, theelectrical conductivity of a conductor structure of the oxygen sensor ismeasured, a first exhaust-gas temperature is determined from thismeasurement, and the first exhaust-gas temperature is compared with asecond exhaust-gas temperature. A component of the sensor which is ofrelevance to the measurement of the oxygen partial pressure ispreferably used to determine the first exhaust-gas temperature. Theexhaust-gas temperature measurement carried out by means of this sensorcomponent and the comparison with a second exhaust-gas temperature makeit possible to determine the light-off temperature of the catalyticconverter. The second exhaust-gas temperature is therefore preferably atemperature which takes account of the increase in temperature of theexhaust gas as a result of the catalytic converter lighting off. Sincethe light-off temperature represents an important performance feature ofthe catalytic converter, its state of ageing can be determined withregard to its ability to catalyse a reaction at an early time. The stateof ageing of the catalytic converter which is characterized by thelight-off temperature can be expressed, for example, by an ageingcharacteristic value. After the internal combustion engine has warmed upor after the light-off temperature of the catalytic converter has beendetermined, it is preferable for the oxygen sensor to be used to measurethe oxygen partial pressure of the internal combustion engine exhaustgas. λ-control for normal operation of the internal combustion enginecan then be effected using this measurement.

[0012] In one embodiment of the method, to determine the firstexhaust-gas temperature the electrical conductivity of a solidelectrolyte, which is used to measure the oxygen partial pressure, ofthe oxygen sensor is measured. This advantageously makes dual use of thesensitive region of the oxygen sensor.

[0013] In a further embodiment of the method, to determine the firstexhaust-gas temperature the electrical conductivity of a heatingconductor structure of the oxygen sensor is measured. Since heating ofthe oxygen sensor is required for the oxygen partial pressuremeasurement function, the oxygen sensor is generally provided with aheating conductor structure. Dual use is advantageously made of thiscomponent of the oxygen sensor. It is preferable for the temperature tobe determined by measuring the conductivity of the heating conductorstructure as the internal combustion engine warms up. After it haswarmed up or after light-off of the catalytic converter has beendetermined by means of the temperature determination, operation of theoxygen sensor is switched over to oxygen partial pressure measurement.

[0014] In a further embodiment of the method, the second exhaust-gastemperature is measured using a temperature probe arranged in theexhaust pipe. With a suitable arrangement of oxygen sensor andtemperature probe, it is possible to record and determine an increase inthe exhaust-gas temperature which is brought about by the catalyticconverter lighting off. It is therefore likewise possible to concludewhether or not the catalytic converter is lighting off late as a resultof ageing and to evaluate the level of catalytic converter ageing withregard to the light-off temperature.

[0015] In a further embodiment of the method, the second exhaust-gastemperature is measured using a second oxygen sensor. For this purpose,the second oxygen sensor is actuated as described above, and the secondexhaust-gas temperature is measured by measuring the conductivity of itsoxygen-sensitive region or its heating conductor structure. It ispreferable for the second oxygen sensor to be arranged in the exhaustpipe upstream of the exhaust-gas catalytic converter and for the otheroxygen sensor to be arranged in the catalytic converter or in theexhaust pipe downstream of the catalytic converter. After light-off ofthe catalytic converter has been recorded, the oxygen sensors are usedto measure the oxygen partial pressure and they can be used, forexample, to perform λ-control.

[0016] In a further embodiment of the method, the second exhaust-gastemperature is determined by modelling. By way of example, expectedvalues for an exhaust-gas temperature at the location of the oxygensensor can be stored in a control unit by comparison measurements withan unaged catalytic converter. In this case, it is possible also to takeinto account the operating point of the internal combustion engine.Conclusions can be drawn as to the light-off temperature and/or thestate of ageing of the catalytic converter by comparison with the firstexhaust-gas temperature determined using the oxygen sensor.

[0017] In a further embodiment of the method, a light-off temperature ofthe catalytic converter is determined from the comparison of the firstexhaust-gas temperature and the second exhaust-gas temperature, and anoxygen storage capacity of the catalytic converter is determined fromthe measurement of the oxygen partial pressure, and the state of ageingof the catalytic converter is determined from the light-off temperatureand the oxygen storage capacity. By way of example, the state of ageingof the catalytic converter, as characterized by the light-offtemperature, can be expressed by a first ageing characteristic value,and the state of ageing of the catalytic converter, as characterized bythe oxygen storage capacity, can be expressed by a second ageingcharacteristic value. The ageing characteristic values can then belevelled out or compared with one another, so that the state of ageingof the catalytic converter can be analysed more reliably and morecomprehensively.

[0018] Other objects, advantages and novel features of the presentinvention will become apparent from the following detailed descriptionof the invention when considered in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 shows a diagrammatically depicted cross section through anoxygen sensor,

[0020]FIG. 2 shows a schematic block diagram of an arrangement of oxygensensors and an exhaust-gas catalytic converter,

[0021]FIG. 3 shows a schematic block diagram of a second arrangement ofoxygen sensors and an exhaust-gas catalytic converter,

[0022]FIG. 4 shows a schematic block diagram of a third arrangement ofoxygen sensors and an exhaust-gas catalytic converter,

[0023]FIG. 5 shows a schematic block diagram of a fourth arrangement ofoxygen sensors and an exhaust-gas catalytic converter,

[0024]FIG. 6 shows a schematic block diagram of a fifth arrangement ofoxygen sensors and an exhaust-gas catalytic converter,

[0025]FIG. 7 shows a schematic block diagram of a sixth arrangement ofoxygen sensors and an exhaust-gas catalytic converter,

[0026]FIG. 8 shows a schematic block diagram of a seventh arrangement ofoxygen sensors and an exhaust-gas catalytic converter,

[0027]FIG. 9 shows a schematic block diagram of an eighth arrangement ofoxygen sensors and an exhaust-gas catalytic converter.

DETAILED DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 diagrammatically depicts a cross section through thestructure of an electrochemical oxygen sensor 1. The oxygen sensor 1 hasan oxygen-sensitive region 2 which is formed, for example, by a solidelectrolyte which contains zirconium dioxide (ZrO₂) and has an oxygenconductivity. The oxygen-sensitive region 2 on one side has a phaseboundary with respect to the exhaust-gas side and on the other side hasa phase boundary with respect to the air side. An exhaust-gas electrode3 and an air electrode 4 are arranged on the solid electrolyte at thesephase boundaries. The electrodes 3, 4 are gas-permeable, so that contactbetween the corresponding gas and the solid electrolyte is possible.Moreover, the oxygen sensor 1 has a heating conductor structure 6 whichis embedded in an insulator 7 and is in thermal contact with theoxygen-sensitive region 2. The electrical feed lines which are requiredto operate the sensor 1 and a connected control unit are notillustrated, for the sake of clarity.

[0029] To measure the oxygen partial pressure in the exhaust gas, thesensor 1 is heated to its operating temperature by current beingsupplied to the heating conductor structure 6. Oxidation or reductionreactions, which lead to a thermodynamic equilibrium being establishedwith regard to the oxygen partial pressure, may occur at the exhaust-gaselectrode 3. Designing the exhaust-gas electrode 3 to contain preciousmetal makes it possible to catalytically assist this operation.Differences between the oxygen partial pressure on the exhaust-gas sideand the known oxygen partial pressure on the air side manifestthemselves in the occurrence of a Nernst voltage which can be tapped offat the electrodes 3, 4. The oxygen sensor 1 can in this case beconsidered an active component in the electrical engineering sense. TheNernst voltage is determined and evaluated by the connected controlunit, so that the oxygen partial pressure in the exhaust gas ismeasured.

[0030] This function of the electrochemical oxygen sensor 1, which isknown per se, is now supplemented according to the invention by atemperature-measuring function. To carry out this function, the heatingremains switched off and a test voltage is applied to the electrodes 3,4, and the level of current flowing across the solid electrolyte ismeasured. This is used to determine the electrical conductivity of theoxygen-sensitive region 2. The oxygen sensor 1 can in this case beconsidered a passive component in the electrical engineering sense. Theelectrical conductivity of the solid electrolyte has a material-specifictemperature dependency which is available to the connected control unitfor analysis. Therefore, the temperature of the solid electrolyte caneasily be determined in the control unit using the electricalconductivity value. The oxygen-sensitive region 2 of the oxygen sensor 1therefore serves as a temperature-sensitive region when the sensor isperforming this function. Since the oxygen sensor is in contact with theexhaust gas, the solid electrolyte of the sensor 1 approximately adoptsthe exhaust-gas temperature, and consequently the exhaust-gastemperature can also be determined from the temperature of the solidelectrolyte. Any temperature losses which may be present at the phaseboundary and in the temperature-sensitive region 2 can be taken intoaccount by advance calibration of the sensor which takes such lossesinto account. It is preferable for the control unit to contain acharacteristic curve which describes the relationship between theelectrical conductivity of the temperature-sensitive region 2 and theexhaust-gas temperature and which is evaluated by the control unit.

[0031] The heating conductor structure 6 can in a similar way likewiseact as a temperature-sensitive region and be used to determine theexhaust-gas temperature. For this purpose, a test voltage is likewiseapplied to the terminals of the heating conductor structure 6, and theflow of current is measured. The electrical conductivity of the heatingconductor structure 6 is determined from the magnitude of the testvoltage and the level of the current flowing. The exhaust-gastemperature is then determined analogously to the procedure describedabove.

[0032] It is advantageous in particular for the exhaust-gas temperatureto be determined both via the electrical conductivity of the solidelectrolyte and via the electrical conductivity of the heating conductorstructure 6. In this operating mode of the oxygen sensor 1, the solidelectrolyte and the heating conductor structure 6 each form atemperature-sensitive region. Therefore, two measured values, which canbe compared with one another and/or levelled out, are obtained for theexhaust-gas temperature. This increases the accuracy of thedetermination of the temperature of the exhaust gas. The accuracy can befurther improved by taking account of the air temperature. For thispurpose, the latter temperature is determined, for example by atemperature sensor (not shown), and is used for correction purposes whendetermining the exhaust-gas temperature.

[0033] The text which follows explains various arrangements of anexhaust-gas catalytic converter and an oxygen sensor which can be usedto determine the state of ageing of the exhaust-gas catalytic converter.

[0034]FIG. 2 shows a catalytic converter 20 which is arranged in anexhaust pipe of an internal combustion engine (not shown), preferablyclose to the internal combustion engine. An exhaust-gas catalyticconverter 21 with an oxygen storage capacity, such as for example athree-way catalytic converter or an oxidation catalytic converter, isprovided in the catalytic converter 20. The direction of flow of theexhaust gas is indicated by arrows. An oxygen sensor 22 and 23 isarranged in the exhaust pipe on the inlet side and the outlet side,respectively, of the exhaust-gas catalytic converter 21. The sensors 22,23 are connected to a control unit 25 via control lines 26. The oxygensensor 23 is designed as described in connection with FIG. 1 and can beactuated and operated by the control unit 25 both to determine theoxygen partial pressure in the exhaust gas and to determine theexhaust-gas temperature, as described. The oxygen sensor 22 can be ofany desired design.

[0035] To determine the state of ageing of the catalytic converter 21,its light-off temperature and its oxygen storage capacity are determinedas described below.

[0036] After the internal combustion engine has started up, the oxygensensor 23, as described above, is operated in a first mode fortemperature measurement. The first exhaust-gas temperature determined inthis way downstream of the catalytic converter is compared by thecontrol unit 25 with a second exhaust-gas temperature which is to beexpected at the location of the oxygen sensor 23. The expected secondexhaust-gas temperature in this case results from a calculation model orfrom characteristic diagrams which are available to the control unit 25.The starting point in this case is advantageously an exhaust-gastemperature on the entry side of the catalytic converter 21, which hasbeen modelled on the basis of the operating parameters of the internalcombustion engine. The model or the characteristic diagrams also takeinto account the fact that additional introduction of heat into theexhaust gas takes place at a predeterminable time as a result of theunaged catalytic converter 21 lighting off. If the first exhaust-gastemperature determined by means of the oxygen sensor remains behind themodelled second exhaust-gas temperature in terms of its magnitude and/orin terms of time, this can be attributed to an increased light-offtemperature induced by ageing. Consequently, the state of ageing of thecatalytic converter 21 can be analysed with regard to its light-offtemperature by comparison of the first exhaust-gas temperature,determined by means of the oxygen sensor 23, with the modelled secondexhaust-gas temperature. In the event of a suitably high deteriorationin the light-off temperature, the control unit 25 can output a signalwhich indicates the increased catalytic converter ageing.

[0037] When the internal combustion engine has warmed up or after apredeterminable exhaust-gas temperature has been reached, the oxygensensors are used for λ-control of the internal combustion engine. Inthis case, the oxygen sensor 22 serves as a control sensor, and theoxygen sensor 23 serves as a trimming sensor or a diagnosis sensor. Withthe aid of the oxygen sensor 22, a λ-control oscillation of definedamplitude and frequency at which the combustion operations in theinternal combustion engine take place is imposed. The procedure fordoing this will be familiar to the person skilled in the art andrequires no further explanation at this point. The λ-oscillations in theexhaust-gas composition which are present on the entry side of thecatalytic converter 21, however, are increasingly smoothed as theexhaust gas passes through the catalytic converter, on account of itsoxygen storage capacity. The result is a decrease in the amplitude ofthe λ-oscillation across the catalytic converter. With a high oxygenstorage capacity, by way of example, it is no longer possible to detectany λ-oscillation on the exit side of the catalytic converter 21. Inthis case, the λ value has levelled out at the constant mean value ofthe λ-oscillation present upstream of the catalytic converter 21. Theamplitude of the λ-oscillation which can be detected after the exhaustgas has passed through a partial section of or the entire catalyticconverter 21 is therefore a measure of the oxygen storage capacity ofthe catalytic converter partial section or of the entire catalyticconverter 21. In chemical equilibrium, however, the λ value is deriveddirectly from the oxygen partial pressure. Therefore, by measuring theoxygen partial pressure using the oxygen sensor 23, it is possible todetermine the λ value at the installation site and therefore also todetermine the oxygen storage capacity of the catalytic converter 21. Tobe more precise, it should be mentioned that the oxygen storage capacityof the catalytic converter 21 can be determined in particular when adetectable amplitude of the λ-oscillation is present. If no suchamplitude is present, it is generally only possible to conclude that theoxygen storage capacity of the catalytic converter 21 has not droppedbelow a defined level. In this case, it can be assumed that thecatalytic converter 21 has not undergone any ageing in terms of itsoxygen storage capacity.

[0038] If the oxygen sensor 23 on the exit side of the catalyticconverter 21 records some degree of amplitude of the λ-oscillation, thismeans that some degree of catalytic converter ageing has occurred.Therefore, the catalytic converter ageing can be determined in themanner outlined by measurement of the oxygen storage capacity. Thisoperating mode of the oxygen sensor 23 is set after the internalcombustion engine has warmed up or after light-off of the catalyticconverter 21 has been recorded. For this purpose, the oxygen sensor 23has to be heated to operating temperature by passing current through theheating conductor structure 6. This makes it available for measurementof the oxygen partial pressure. Temperature determination by means ofthe oxygen sensor 23 is now no longer desired.

[0039]FIG. 3 shows a further advantageous arrangement for realizing theinvention. Components which have substantially the same effect in thisfigure and in the following figures are denoted by the same referencenumerals as in FIG. 2. Unlike in the arrangement shown in FIG. 2, inthis case a temperature probe 24 is additionally provided in the exhaustpipe on the entry side of the catalytic converter 21. The temperatureprobe 24 is likewise connected to the control unit 25 via a control line26.

[0040] Analogously to the procedure corresponding to the arrangementshown in FIG. 2, the state of ageing of the catalytic converter isassessed with regard to its light-off temperature by comparing the firstexhaust-gas temperature, determined by means of the oxygen sensor 23,with the modelled second exhaust-gas temperature. The light-off of thecatalytic converter 21 can also be monitored directly by forming thedifference between the first exhaust-gas temperature, determined bymeans of the oxygen sensor 23, and the exhaust-gas temperaturedetermined by means of the temperature probe 24. If a temperaturedifference does not occur to the same extent, and/or occurs at a latertime, compared to the temperature difference which would be expected inan unaged catalytic converter 21, an ageing-induced deterioration in thecatalytic converter action is observed and is evaluated accordingly. Ofcourse, it is also possible to determine ageing of the catalyticconverter 21 with regard to its oxygen storage capacity, as in thearrangement illustrated in FIG. 2, and this step is carried out afterthe internal combustion engine has warmed up or after a predeterminableexhaust-gas temperature has been reached, as described. The same alsoapplies to the λ-control of the internal combustion engine operation.With this temperature probe 24, the exhaust-gas temperature upstream ofthe catalytic converter 21 can be determined more accurately than bymodelling. Therefore, the modelling of the exhaust-gas temperature onthe exit side of the catalytic converter leads to a more accurate andmore reliable result.

[0041]FIG. 4 shows a further advantageous arrangement used to implementthe invention. Unlike in FIG. 3, the temperature probe 24 or itstemperature-sensitive part is arranged in the catalytic converter 21.The procedure used to determine the catalytic converter ageing to a verylarge extent corresponds to the procedure which has been explained inconnection with the arrangement shown in FIG. 3. Furthermore, thearrangement shown in FIG. 4 makes it possible specifically to analysethe catalytic converter part located downstream of the temperaturesensor 24. Exothermic reactions which occur in this downstream catalyticconverter part can be recorded using this arrangement by measuring thetemperature difference. If such reactions are recorded, this is anindication of catalytic converter ageing, since in an unaged catalyticconverter the reactions take place primarily in the upstream region.However, on account of the thermal loading, catalytic converter ageingalso occurs earlier in the upstream region of the catalytic converter21. Therefore, with the arrangement shown in FIG. 4, it is possible toreliably recognize catalytic converter ageing with regard to thelight-off temperature. Determination of ageing of the catalyticconverter 21 with regard to its oxygen storage capacity and λ-control ofthe internal combustion engine operation are carried out in the same wayas described above.

[0042]FIG. 5 shows a further advantageous arrangement for realizing theinvention. Unlike in the arrangement illustrated in FIG. 2, in this casethe oxygen sensor 23 or its temperature-sensitive region is arranged inthe catalytic converter 21. The catalytic converter ageing is determinedanalogously to the arrangement illustrated in FIG. 2. However, bydetermining the temperature in the catalytic converter 21, it ispossible to evaluate the efficiency of the catalytic converter partupstream of the oxygen sensor 23 more accurately. Since ageing phenomenapreferentially occur in the upstream region of the catalytic converter21, it is therefore possible to reliably detect catalytic converterageing with regard to the light-off temperature. The same is also trueof the determination of the ageing of the catalytic converter 21 withregard to its oxygen storage capacity, which is carried out as hasalready been explained above. Since the arrangement of the oxygen sensor23 in the catalytic converter 21 means that only that part of thecatalytic converter which lies upstream of the sensor 23 is recorded, inthis case imposed λ-oscillations occur to a greater extent and thedetermination of the ageing of the catalytic converter 21 with regard toits oxygen storage capacity is more sensitive and more accurate.

[0043]FIG. 6 shows a further advantageous arrangement for realizing theinvention. Unlike in FIG. 5, in this case a temperature probe 24 isadditionally arranged in the exhaust pipe on the entry side of thecatalytic converter 21. This temperature probe 24 allows the exhaust-gastemperature upstream of the catalytic converter 21 to be determined moreaccurately than by modelling. This leads to a more reliable result whendetermining the state of ageing of the catalytic converter 21 inaccordance with the procedure used in the arrangement illustrated inFIG. 5. With the arrangement shown in FIG. 6, it is possible to directlydetermine an increase in the exhaust-gas temperature caused by reactionsin the upstream part of the catalytic converter by differencemeasurement. This allows reliable evaluation of the state of ageing ofthe catalytic converter 21. The ageing of the catalytic converter 21with regard to its oxygen storage capacity is determined in the same wayas in the arrangement illustrated in FIG. 5.

[0044]FIG. 7 shows a further advantageous arrangement for realizing theinvention. Unlike in the arrangement illustrated in FIG. 6, in this casethe temperature probe 24 is arranged in the catalytic converter 21,upstream of the oxygen sensor 23. With this arrangement, it is possible,by means of a temperature difference measurement, to monitor a catalyticconverter partial region, preferably in the front half of the catalyticconverter 21. The catalytic converter ageing with regard to thelight-off temperature of the catalytic converter 21 then relatesprimarily to this partial region of the catalytic converter. The ageingof the catalytic converter 21 with regard to its oxygen storage capacityis determined in the same way as in the arrangements illustrated inFIGS. 5 and 6.

[0045]FIG. 8 shows a further advantageous arrangement for realizing theinvention. Unlike in the arrangement illustrated in FIG. 7, in this casethe temperature probe 24 is arranged in the exhaust pipe on the exitside of the catalytic converter 21 and downstream of the oxygen sensor23. On the one hand, a temperature difference measurement can be carriedout with the aid of the oxygen sensor 23 and the temperature probe 24.In this case, the catalytic converter part lying downstream of theoxygen sensor 23 is evaluated with regard to its catalytic activity. Onthe other hand, it is possible to work on the basis of an exhaust-gastemperature which is present on the entry side of the catalyticconverter 21 and is obtained by modelling as explained above. In thiscase, the temperature difference determination allows the entirecatalytic converter 21 to be evaluated integrally with regard to itscatalytic activity and its ageing. The procedure in principlecorresponds to that used in the arrangement illustrated in FIG. 4. Withregard to the catalytic converter ageing in terms of the oxygen storagecapacity, the statements which have been made in connection with thearrangement illustrated in FIG. 7 apply in this case too.

[0046]FIG. 9 shows a further advantageous arrangement for realizing theinvention. The temperature probe 24 is in this case likewise arrangeddownstream of the oxygen sensor 23, but unlike in the design illustratedin FIG. 8 is arranged in the downstream region of the catalyticconverter 21. The procedure used to determine the catalytic converterageing largely corresponds to that used in the arrangement shown in FIG.8. However, on account of the fact that the temperature probe 24 isarranged in the catalytic converter, only the exothermic characteristicsof an upstream catalytic converter volume are recorded. Determination ofthe ageing of the catalytic converter with regard to its oxygen storagecapacity takes place as in the arrangements illustrated in FIGS. 5 to 8.

[0047] A further improvement to the determination of the catalyticconverter ageing with regard to the light-off temperature is achieved byusing an oxygen sensor 22 which is of similar construction to the oxygensensor 23. This is then likewise used to determine the temperatureduring the warm-up phase of the internal combustion engine. Togetherwith a temperature probe 24 arranged between the oxygen sensors 22, 23,three temperatures determined at different locations are then available.The locations can be selected appropriately with a view to acquiringinformation which is as accurate as possible, with the oxygen sensor 22being arranged upstream of the catalytic converter 21 for reasons ofλ-control. The oxygen sensor 23 and the temperature probe 24 may bearranged both in the catalytic converter 21 and also in the exhaust pipeon the exit side of the catalytic converter 21. This allows accurateposition resolution when determining the activity of the catalyticconverter 21 and therefore accurate and reliable determination of itsstate of ageing. However, if appropriate it is also possible to make dowithout the temperature probe 24.

[0048] As has been explained, the device according to the invention andthe method according to the invention can be used to determine andmonitor the state of ageing of a catalytic converter both with regard toits light-off temperature and with regard to its oxygen storagecapacity. Since two different properties of the catalytic converter arebeing recorded, and these properties are subject to different ageinginfluences, this allows comprehensive and reliable evaluation of thecatalytic converter ageing. The ageing with regard to the light-offtemperature can be evaluated separately from the oxygen storagecapacity. By way of example, it is possible to provide for a warningsignal to be activated for both properties. It is in each caseadvantageous to introduce an ageing characteristic value, for examplerelated to a maximum permissible ageing. The two separately determinedcharacteristic values may, however, also be combined to form a commonageing characteristic value, and a signal can be output when a definablelevel of ageing is exceeded.

[0049] The foregoing disclosure has been set forth merely to illustratethe invention and is not intended to be limiting. Since modifications ofthe disclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. Device for determining a state of ageing of anexhaust-gas catalytic converter arranged in an exhaust pipe of aninternal combustion engine, having an oxygen sensor which is arranged inthe exhaust pipe and is assigned to the exhaust-gas catalytic converter,the oxygen sensor having an oxygen-sensitive region for measuring anoxygen partial pressure in the exhaust gas and being connected to anelectronic control unit, wherein the oxygen sensor has atemperature-sensitive region and can be actuated by the control unit insuch a manner that at least one of a temperature measurement and anoxygen partial pressure measurement can be carried out.
 2. Deviceaccording to claim 1, wherein the temperature-sensitive region of theoxygen sensor is formed by the oxygen-sensitive region and is a solidelectrolyte.
 3. Device according to claim 1, wherein thetemperature-sensitive region of the oxygen sensor is a heating conductorstructure.
 4. Device according to claim 1, wherein a temperature probeis provided in the exhaust pipe, and the temperature probe and theoxygen sensor are arranged in such a manner in the exhaust pipe that atleast a partial region of the exhaust-gas catalytic converter is locatedbetween the oxygen sensor and the temperature probe.
 5. Device accordingto claim 2, wherein a temperature probe is provided in the exhaust pipe,and the temperature probe and the oxygen sensor are arranged in such amanner in the exhaust pipe that at least a partial region of theexhaust-gas catalytic converter is located between the oxygen sensor andthe temperature probe.
 6. Device according to claim 3, wherein atemperature probe is provided in the exhaust pipe, and the temperatureprobe and the oxygen sensor are arranged in such a manner in the exhaustpipe that at least a partial region of the exhaust-gas catalyticconverter is located between the oxygen sensor and the temperatureprobe.
 7. Device according to claim 1, wherein the oxygen sensor isarranged in one of the exhaust-gas catalytic converter and the exhaustpipe downstream of the exhaust-gas catalytic converter, and a secondoxygen sensor is arranged in the exhaust pipe upstream of theexhaust-gas catalytic converter.
 8. Device according to claim 2, whereinthe oxygen sensor is arranged in one of the exhaust-gas catalyticconverter and the exhaust pipe downstream of the exhaust-gas catalyticconverter, and a second oxygen sensor is arranged in the exhaust pipeupstream of the exhaust-gas catalytic converter.
 9. Device according toclaim 3, wherein the oxygen sensor is arranged in one of the exhaust-gascatalytic converter and the exhaust pipe downstream of the exhaust-gascatalytic converter, and a second oxygen sensor is arranged in theexhaust pipe upstream of the exhaust-gas catalytic converter.
 10. Deviceaccording to claim 4, wherein the oxygen sensor is arranged in one ofthe exhaust-gas catalytic converter and the exhaust pipe downstream ofthe exhaust-gas catalytic converter, and a second oxygen sensor isarranged in the exhaust pipe upstream of the exhaust-gas catalyticconverter.
 11. Method for determining a state of ageing of anexhaust-gas catalytic converter arranged in an exhaust pipe of aninternal combustion engine, in which an oxygen partial pressure of theexhaust gas is determined using an oxygen sensor assigned to theexhaust-gas catalytic converter, wherein, as the internal combustionengine is warming up, an electrical conductivity of a conductorstructure of the oxygen sensor is measured, a first exhaust-gastemperature is determined from this measurement, and the firstexhaust-gas temperature is compared with a second exhaust-gastemperature.
 12. Method according to claim 11, wherein, to determine thefirst exhaust-gas temperature, the electrical conductivity of a solidelectrolyte, which is used to determine the oxygen partial pressure, ofthe oxygen sensor is measured.
 13. Method according to claim 11,wherein, to determine the first exhaust-gas temperature, the electricalconductivity of a heating conductor structure of the oxygen sensor ismeasured.
 14. Method according to claim 11, wherein the secondexhaust-gas temperature is measured using a temperature probe arrangedin the exhaust pipe.
 15. Method according to claim 12, wherein thesecond exhaust-gas temperature is measured using a temperature probearranged in the exhaust pipe.
 16. Method according to claim 13, whereinthe second exhaust-gas temperature is measured using a temperature probearranged in the exhaust pipe.
 17. Method according to claim 11, whereinthe second exhaust-gas temperature is measured using a second oxygensensor.
 18. Method according to claim 12, wherein the second exhaust-gastemperature is measured using a second oxygen sensor.
 19. Methodaccording to claim 13, wherein the second exhaust-gas temperature ismeasured using a second oxygen sensor.
 20. Method according to claim 11,wherein the second exhaust-gas temperature is determined by modelling.21. Method according to claim 12, wherein the second exhaust-gastemperature is determined by modelling.
 22. Method according to claim13, wherein the second exhaust-gas temperature is determined bymodelling.
 23. Method according to claim 11, wherein a light-offtemperature of the catalytic converter is determined from the comparisonof the first exhaust-gas temperature and the second exhaust-gastemperature, and an oxygen storage capacity of the catalytic converteris determined from the measurement of the oxygen partial pressure, andthe state of ageing of the catalytic converter is determined from thelight-off temperature and the oxygen storage capacity.
 24. Methodaccording to claim 12, wherein a light-off temperature of the catalyticconverter is determined from the comparison of the first exhaust-gastemperature and the second exhaust-gas temperature, and an oxygenstorage capacity of the catalytic converter is determined from themeasurement of the oxygen partial pressure, and the state of ageing ofthe catalytic converter is determined from the light-off temperature andthe oxygen storage capacity.
 25. Method according to claim 13, wherein alight-off temperature of the catalytic converter is determined from thecomparison of the first exhaust-gas temperature and the secondexhaust-gas temperature, and an oxygen storage capacity of the catalyticconverter is determined from the measurement of the oxygen partialpressure, and the state of ageing of the catalytic converter isdetermined from the light-off temperature and the oxygen storagecapacity.
 26. Method according to claim 14, wherein a light-offtemperature of the catalytic converter is determined from the comparisonof the first exhaust-gas temperature and the second exhaust-gastemperature, and an oxygen storage capacity of the catalytic converteris determined from the measurement of the oxygen partial pressure, andthe state of ageing of the catalytic converter is determined from thelight-off temperature and the oxygen storage capacity.
 27. Methodaccording to claim 15, wherein a light-off temperature of the catalyticconverter is determined from the comparison of the first exhaust-gastemperature and the second exhaust-gas temperature, and an oxygenstorage capacity of the catalytic converter is determined from themeasurement of the oxygen partial pressure, and the state of ageing ofthe catalytic converter is determined from the light-off temperature andthe oxygen storage capacity.
 28. Method according to claim 16, wherein alight-off temperature of the catalytic converter is determined from thecomparison of the first exhaust-gas temperature and the secondexhaust-gas temperature, and an oxygen storage capacity of the catalyticconverter is determined from the measurement of the oxygen partialpressure, and the state of ageing of the catalytic converter isdetermined from the light-off temperature and the oxygen storagecapacity.
 29. An apparatus for determining a state of ageing of anexhaust-gas catalytic converter arranged in an exhaust pipe of aninternal combustion engine, comprising: an oxygen sensor disposed in theexhaust pipe, the oxygen sensor having an oxygen-sensitive region fordetecting an oxygen partial pressure in the exhaust gas and atemperature-sensitive region for detecting a temperature of the exhaustgas; and a control unit adapted to receive signals from the oxygensensor corresponding to a detected oxygen partial pressure and adetected exhaust gas temperature, wherein the control unit determines atleast one of the exhaust gas temperature and the oxygen partial pressurefrom at least one of the oxygen sensor signals.
 30. The apparatus ofclaim 29, wherein the temperature-sensitive region and theoxygen-sensitive region are formed from a single region of a solidelectrolyte.
 31. The apparatus of claim 29, wherein thetemperature-sensitive region is a heating conductor structure.
 32. Theapparatus of claim 29, further comprising: a temperature probe providedin the exhaust pipe, wherein at least a partial region of the catalyticconverter is located between the oxygen sensor and the temperatureprobe.
 33. The apparatus of claim 30, further comprising: a temperatureprobe provided in the exhaust pipe, wherein at least a partial region ofthe catalytic converter is located between the oxygen sensor and thetemperature probe.
 34. The apparatus of claim 31, further comprising: atemperature probe provided in the exhaust pipe, wherein at least apartial region of the catalytic converter is located between the oxygensensor and the temperature probe.
 35. The apparatus of claim 29, furthercomprising: a second oxygen sensor disposed in the exhaust pipe upstreamof the catalytic converter, wherein the first oxygen sensor is disposedin one of the catalytic converter and the exhaust pipe downstream of thecatalytic converter.
 36. The apparatus of claim 30, further comprising:a second oxygen sensor disposed in the exhaust pipe upstream of thecatalytic converter, wherein the first oxygen sensor is disposed in oneof the catalytic converter and the exhaust pipe downstream of thecatalytic converter.
 37. The apparatus of claim 31, further comprising:a second oxygen sensor disposed in the exhaust pipe upstream of thecatalytic converter, wherein the first oxygen sensor is disposed in oneof the catalytic converter and the exhaust pipe downstream of thecatalytic converter.
 38. The apparatus of claim 32, further comprising:a second oxygen sensor disposed in the exhaust pipe upstream of thecatalytic converter, wherein the first oxygen sensor is disposed in oneof the catalytic converter and the exhaust pipe downstream of thecatalytic converter.
 39. A method for determining a state of ageing ofan exhaust-gas catalytic converter arranged in an exhaust pipe of aninternal combustion engine, comprising the steps of: providing oxygensensor disposed in the exhaust pipe, the oxygen sensor having anoxygen-sensitive region for detecting an oxygen partial pressure in theexhaust gas and a temperature-sensitive region for detecting atemperature of the exhaust gas; detecting an oxygen partial pressure ofthe exhaust gas; detecting a first exhaust-gas temperature bydetermining an electrical conductivity of a conductor structure of theoxygen sensor as the internal combustion engine is warming up; andcomparing the first exhaust-gas temperature with a second exhaust-gastemperature.
 40. The method of claim 39, wherein the step of detectingthe first exhaust-gas temperature comprises determining the electricalconductivity of a solid electrolyte used to determine the oxygen partialpressure.
 41. The method of claim 39, wherein, the step of detecting thefirst exhaust-gas temperature comprises determining the electricalconductivity of a heating conductor structure of the oxygen sensor. 42.The method of claim 39, wherein the second exhaust-gas temperature isdetected using a temperature probe arranged in the exhaust pipe.
 43. Themethod of claim 40, wherein the second exhaust-gas temperature isdetected using a temperature probe arranged in the exhaust pipe.
 44. Themethod of claim 41, wherein the second exhaust-gas temperature isdetected using a temperature probe arranged in the exhaust pipe.
 45. Themethod of claim 39, wherein the second exhaust-gas temperature isdetected using a second oxygen sensor.
 46. The method of claim 40,wherein the second exhaust-gas temperature is detected using a secondoxygen sensor.
 47. The method of claim 41, wherein the secondexhaust-gas temperature is detected using a second oxygen sensor. 48.The method of claim 39, wherein the second exhaust-gas temperature isdetermined by modelling.
 49. The method of claim 40, wherein the secondexhaust-gas temperature is determined by modelling.
 50. The method ofclaim 41, wherein the second exhaust-gas temperature is determined bymodelling.
 51. The method of claim 39, further comprising the steps of:determining a light-off temperature of the catalytic converter bycomparing the first exhaust-gas temperature and the second exhaust-gastemperature; determining an oxygen storage capacity of the catalyticconverter from the measurement of the oxygen partial pressure; anddetermining the state of ageing of the catalytic converter from thelight-off temperature and the oxygen storage capacity.
 52. The method ofclaim 40, further comprising the steps of: determining a light-offtemperature of the catalytic converter by comparing the firstexhaust-gas temperature and the second exhaust-gas temperature;determining an oxygen storage capacity of the catalytic converter fromthe measurement of the oxygen partial pressure; and determining thestate of ageing of the catalytic converter from the light-offtemperature and the oxygen storage capacity.
 53. The method of claim 41,further comprising the steps of: determining a light-off temperature ofthe catalytic converter by comparing the first exhaust-gas temperatureand the second exhaust-gas temperature; determining an oxygen storagecapacity of the catalytic converter from the measurement of the oxygenpartial pressure; and determining the state of ageing of the catalyticconverter from the light-off temperature and the oxygen storagecapacity.
 54. The method of claim 42, further comprising the steps of:determining a light-off temperature of the catalytic converter bycomparing the first exhaust-gas temperature and the second exhaust-gastemperature; determining an oxygen storage capacity of the catalyticconverter from the measurement of the oxygen partial pressure; anddetermining the state of ageing of the catalytic converter from thelight-off temperature and the oxygen storage capacity.
 55. The method ofclaim 43, further comprising the steps of: determining a light-offtemperature of the catalytic converter by comparing the firstexhaust-gas temperature and the second exhaust-gas temperature;determining an oxygen storage capacity of the catalytic converter fromthe measurement of the oxygen partial pressure; and determining thestate of ageing of the catalytic converter from the light-offtemperature and the oxygen storage capacity.
 56. The method of claim 44,further comprising the steps of: determining a light-off temperature ofthe catalytic converter by comparing the first exhaust-gas temperatureand the second exhaust-gas temperature; determining an oxygen storagecapacity of the catalytic converter from the measurement of the oxygenpartial pressure; and determining the state of ageing of the catalyticconverter from the light-off temperature and the oxygen storagecapacity.